1. Field of the Invention
The present invention relates to an infrared sensor that detects infrared rays.
2. Description of Related Art
Infrared sensors are largely classified into thermal-type infrared sensors that make use of a temperature change of a thin film due to reception of infrared rays and quantum-type infrared sensors that make use of an electrical phenomenon (photoelectric conversion) that occurs in a semiconductor material due to light energy of infrared rays. Thermal-type infrared sensors, though being low in detection sensitivity and response speed in comparison to quantum-type infrared sensors, are wide in detection wavelength bandwidth, can be used at room temperature, and are thus widely used in thermography, radiation thermometers, etc.
As thermal-type infrared sensors, thermopiles that detect infrared rays by a thermoelectric effect, and pyroelectric sensors (pyrosensors) that detect infrared rays by a pyroelectric effect are known.
A thermopile has a structure in which a thin film is formed on a substrate and a thermocouple is, disposed on the thin film. A cavity is formed in the substrate, and the thin film is disposed so as to close the cavity from a top surface side of the substrate. The thermocouple has its hot junction and cold junction thermally isolated by the hot junction being disposed at a position opposite to the cavity and the cold junction being disposed at a position opposite to a portion of the substrate at a periphery of the cavity. When a temperature of the thin film rises due to infrared rays, a temperature difference arises between the hot junction and the cold junction of the thermocouple, and a voltage that is in accordance with the temperature difference is output as a detection signal.
With the thermopile, the same materials as materials used in an IC chip or other semiconductor device is used. That is, for example, silicon (Si), silicon oxide (SiO2), and polysilicon are used as the materials of the substrate, the thin film, and the thermocouple. The thermopile can thus be formed on the same substrate as a circuit for processing its detection signal, etc.
A pyroelectric sensor has a structure where a ceramic (bulk ceramic) formed by sintering a powder raw material is sealed inside a package. The ceramic is made to have an intrinsic polarization by a polarization treatment. When a temperature of the ceramic rises due to infrared rays, the intrinsic polarization of the ceramic changes according to the temperature, charges arise on a surface of the ceramic, and a flow of the charges (pyroelectric current) is output as a detection signal.